For glass substrates for various displays, alkali-free glass containing substantially no alkali metal component is required. Alkali-free glass to be used for a display substrate, is usually required to be melted at a high temperature as compared with common soda-lime glass. This is because silica raw material having a high melting point is used in a large amount, or an alkali component having a function to accelerate melting of silica raw material is not used.
Further, a glass substrate for display is required to have a high quality such that it does not have defects influential over display presentation, such as bubbles, striae, non-molten substances, pits, flaws, etc., inside or on the surface of the glass substrate. However, in alkali-free glass to be used for a display substrate, homogeneity in the glass composition tends to be low as compared with common soda-lime glass. In order to obtain glass having high homogeneity in the glass composition, it is effective to finely pulverize glass raw material not to let non-molten raw material remain. However, in the case of using finely pulverized glass raw material, at the time of putting the raw material into a glass melting furnace, the raw material is likely to scatter, whereby there may be such a problem that the glass composition tends to be unstable, or a part of the glass raw material is wasted.
As a method to solve the problem of scattering of finely pulverized glass raw material, a method is known wherein the glass raw material is granulated into granules for use. For example, Patent Document 1 discloses a method for producing granules by using caustic soda or water glass as a binder. Patent Document 2 discloses a method for producing granules by using boric acid as a binder.
Example 1, etc. in Patent Document 3 discloses a method wherein water is added to a mixture containing Portland cement or alumina cement as the main component, followed by kneading and forming into a suitable size and then sintering at from 900 to 1,100° C. to obtain a sintered solid.
Patent Document 3 discloses that alumina cement is used as the glass raw material, and the alumina cement has a composition comprising from 3 to 6 mass % of SiO2, from 49 to 57 mass % of Al2O3, from 0.3 to 3 mass % of Fe2O3, from 35 to 40 mass % of CaO and from 0 to 1 mass % of MgO. The alumina cement having this composition contains CaO.Al2O3 as the main component and does not contain CaO.2Al2O3.